The overall goal of the research is to test an epidemiological hypothesis, the Gateway Hypothesis, in molecular terms. The hypothesis describes the sequence of steps whereby use of one class of drug, e.g. cigarettes (nicotine), precedes the use of other drugs, such as cocaine. We propose to test the Gateway Hypothesis at the molecular level using a mouse model. Our approach is further based on the evidence that addiction shares molecular steps and molecular logic with long-term memory. We will address four specific aims: (1) To determine on a behavioral and transcriptional level whether there is the sequential order between nicotine and cocaine predicted by the Gateway Hypothesis, whether this sequence is unidirectional (from nicotine to cocaine) or bidirectional (from cocaine to nicotine), and whether nicotine can also enhance the response to drugs of abuse other that cocaine, such as morphine. (2) To determine the molecular mechanisms by which nicotine primes an animal to the effects of cocaine, we propose to use gene-chip analysis to selected target regions to survey patterns of mRNA in the striatum, particularly the nucleus accumbens, and the amygdala. We will use pharmacological analysis to dissect any possible signal transduction pathway that may mediate the priming effects, and genetically modified mice to explore some of the selected candidate genes emerging from the screen. (3) To identify molecular mechanisms that might contribute to maintaining the gateway effect, we will examine persistent histone acetylation focusing on promoters of FosB and c-Fos, using chromatin immunoprecipitation assays. (4) To characterize the nature and consequences of adolescent drug exposure in both behavioral and molecular terms, we will replicate selected experiments outlined in Aims 1-3 (a) on adolescent mice to compare these results to those obtained on adults, and (b) on adult mice preexposed to nicotine or cocaine to identify the consequences of adolescent drug exposure for drug responses in adulthood to the same or different drugs. Preliminary results from pilot studies support the proposed approach. In addition to testing certain fundamental hypotheses for understanding addiction, these molecular insights are potentially useful for two reasons: (1) They may provide new molecular targets for the treatment of addiction;(2) They are likely to provide new hypotheses about drug behavior in human populations that can be further explored in epidemiological data. Drug abuse represents one of the most important public health issues in the nation. Drug abuse begins in adolescence, with the use of one of the legal drugs, alcohol or tobacco (nicotine);those who use one of these drugs are at much greater risk of progressing to the use of illicit drugs, such as marijuana or cocaine. Those who start smoking in adolescence are much more likely to continue smoking as adults and to experience the adverse health consequences of smoking than individuals who start smoking later in life. The proposed research aims to elucidate the biological mechanisms that underlie the progression in drug use from nicotine to cocaine by using a mouse model. The findings of the research may make it possible to develop new drugs to prevent and treat substance abuse.